1. Verify Power at the Coil's Positive Terminal:
* With the Key ON (engine not cranking): Use a test light or multimeter to check for battery voltage (+12V) at the positive (+) terminal of the coil. If there's no power here, the problem is upstream of the coil.
2. Investigate the Upstream Power Supply:
* Ignition Switch: A faulty ignition switch can prevent power from reaching the coil. This is a common culprit. You'll need a wiring diagram to trace the power wire from the ignition switch to the coil.
* Ignition Control Module (ICM): The 2.5L engine uses an ICM (sometimes integrated into the distributor, but possibly separate in your year). The ICM controls the ignition timing and provides the power signal to the coil. A faulty ICM is a very likely suspect. It's usually located near the distributor.
* Wiring Harness: Carefully inspect the wiring harness between the ignition switch, ICM (if separate), and the coil for broken wires, corrosion, or loose connections. Pay close attention to the ground wires as well.
* Fuses and Relays: Check the fuses and relays related to the ignition system (consult your owner's manual or a wiring diagram). A blown fuse or faulty relay can interrupt the power supply.
3. Check the Coil's Negative Terminal (Trigger):
* With the Key ON, and while cranking: This terminal receives a pulsed ground signal from the ICM. You'll need a helper or a noid light (a specialized tool for testing ignition signals) to check for this pulse. A multimeter can be used, but you need to be very careful. A lack of signal here means the ICM or its connections aren't functioning properly.
4. Consider the Crankshaft Position Sensor (CKP):
* The CKP sensor tells the ICM the position of the crankshaft. A faulty CKP prevents the ICM from sending the correct signal to the coil. This is less likely if the engine cranks normally, but it's a possibility to consider.
Troubleshooting Tips:
* Wiring Diagram: Obtain a wiring diagram for your specific year and model Jeep Wrangler. This is essential for tracing the electrical paths. A Haynes or Chilton manual can be helpful.
* Systematic Approach: Work your way back from the coil, checking for power and signals at each point in the circuit.
* Ground Connections: Poor ground connections are a common cause of electrical problems. Make sure all ground points in the ignition system are clean and secure.
If you're not comfortable working with automotive electrical systems, take your Jeep to a qualified mechanic. Improper diagnosis and repair can lead to further damage. The intermittent nature of electrical issues can make them challenging to troubleshoot, even for experienced mechanics.
